Tape Eros trouble

[xcortes]

I finished my preamp on Saturday and this time I'm having some troubles that I'd like help addressing.

To go in order the first thing is that the resistance checks are not where they need to be. The first one that I'm not getting is the positions that are supposed to be 130k ohms in the manual. I'm getting 200k. From studying the schematic the only connection I see from that point to ground is a 200k resistor. The other connections are through caps. What am I missing?

Thanks,

[Doc B.]

I suspect that's one of those deals where some meters will be affected by the caps charging and read a little lower, and other meters won't. If it was reading close to 0 or reading infinity I would be concerned. But 130K is pretty close to 200K and probably OK. Try swapping the test leads and see if the reading changes.

[xcortes]

Yeah, that's what I thought. I did swap the test leads without changes.

So that leads to the second question. Voltages are OK (225v and 100v spot on, the other

[Doc B.]

So the hum is there with the inputs shorted? Is it soft and mostly 60Hz or is it buzzy and mostly 120Hz?

[xcortes]

Yes. It's there with the inputs shorted. I'd say hum and 60hz. Maybe it's time to undust my scope?

[Doc B.]

Is the Eros near any other gear? It's fairly susceptible to picking up hum from the power transformer of nearby equipment.

[xcortes]

It's close to a Paramount. I've run my repro at the same position without any noise pick up though.

I'll try it tonight in an isolated position.

[Doc B.]

The Repro is better shielded by virtue of its cabinet, so it's possible that the input circuitry of the Eros is more affected by the PT-4 than the Repro was.

(BTW, PJ and I worked on the design for your TL-404 project this afternoon)

[xcortes]

I spent some time on it. First of all I was wrong when I said the noise came from both channels. The brain can be misleading 'cause when I listen to both channels either through speakers or headphones I'm sure I hear the noise on both sides. But when listening to one at a time it's obvious one channel (the right one) is silent.

So I proceeded to reheat every joint on the offender side but still no improvement. Other test I did was swap the cables coming from the regulator to discard a problem upstream. The noise stayed in the same channel.

I'm sure the problem doesn't have to do with the C4S boards because it was there already before I switched the boards (before I noticed the phono and tape were not the same).

I'll try reheating once again in the next days with less tired eyes and mind.

Thanks Doc

PJ and I worked on the design for your TL-404 project this afternoon

That's so cool! Thanks again guys.

[Paul Joppa]

Swap tubes (if you haven't).

Check cable grounds to power amps.

[Doc B.]

Yeah what PJ says. EF 86s are easy, just swap side to side. Try a different 6922 also, as it could be that one side of the one you have in there is noisy.

[xcortes]

I've already tried two sets of tubes and with each set swaped EF 86s from side to side. Both sets are silent on the Repro so I think (unfortunately) that I can rule tube problems out.

[Doc B.]

OK, you've done a good process of elimination. Shunt reg is OK, C4S's are OK, tube are OK. That leaves the EQ section. Does the hum change character at all when you switch EQ? if so, the problem may be nearer the EF86 parts of the circuit, before the EQ changes the bass contour. If the hum doesn't change character it may be that the problem is around the 6922 part of the circuit, after the bass EQ bits.

[xcortes]

Lot's of time into it today without success. First, following Doc's suggestion I tried to see if the character of the noise changes with the eq. Apparently it doesn't. But then I checked if the noise is still there with the input to the 6922s shorted. It disappears!

So I decided to look at everything again. I took the C4S out to review all connections and followed the instructions to review that everything is connected to where it's supposed to be. Also visually compared both channels. All looks good.

So I put the C4s back in after (again) resoldering everything. I switched the C4S boards too be sure. Problem remains.

Again swapped the regulated B+ wires coming from the shunt reg. Noise remains at the same channel.

Finally I took out the Auricaps and installed the stock caps (you can see how desperate I am!) and, obviously, nothing happened.

I guess I wasn't careful enough when soldering back the C4S boards 'cause now a have a slight buzz on the good channel :>)

I will leave the amp alone for a few days and then I'll probably replace all the signal wires.

I've never been conquered by a BH amp but right now I don't see how I can win this battle.

[Doc B.]

Whoa! Those 6922 grids are connected to the EF86 plates and the 6922 cathode is sitting at close to 100V. Don't short them out.

I'm still wondering if the problem is at the eq switch. Maybe we should send you another and you can try replacing it.